What i am working on with my 67 cuda is making 1/16" steel reinforcing plates shaped like how i drew on your pic. These will be placed on the inside or backside of the door jamb and spot welded in place to reinforce the thin steel, then the floating nutplates for the striker pin will get welded to the new reinforcement plates. Replace old saggy door hinges while your at it, as a dropping door catch smacking the striker pin is part of the problem